In recent years, in fields, such as automobiles, construction materials, household appliances, and so on, the use of high-tensile steel sheet has increased. Particularly in the automotive industry, the adoption of high-tensile steel sheet is increasing rapidly with a view to reducing car weight for better fuel economy and improved crashworthiness.
In addition to high tensile strength, to provide ductility to withstand a complicated press-forming, development has been actively pushed forward for a high-ductile high-tensile steel of a structure containing retained austenite to utilize a strain-induced transformation of this retained austenite.
As an example of high-tensile steel sheets, there is one that has a composition including addition elements such as Si, Mn, Ti, Al and P, which is disclosed in Japanese Patent Publication No. 3-51778. It is well known, however, that as the Si content is increased, an Si oxide film is formed on the surfaces of the steel sheet during an annealing process, thus deteriorating the steel properties, such as chemical properties, electro-galvanized coating adhesion, hot-dip galvanizability, and hot-dip galvanized coating adhesion. Above all else, the big problem is the inferior hot-dip galvanizability of Si-containing high-tensile steel sheets, i.e., the applied zinc does not adhere to some portions of the steel sheet (so-called “bare-spot”) in hot-dip galvanizing, or the adhesion of the coating is insufficient. When a steel sheet contains not less than 0.1% carbon by mass, there has been difficulty in carrying out galvanizing or forming a stable galvanized layer even on a continuous hot-dip galvanizing line, which includes a RTH (all Radiant Tube Heating) type furnace or a NOF (Non Oxidizing Furnace) type furnace.
As a method for increasing ductility and tensile strength without increasing the Si content, a technique for achieving high ductility and high tensile strength has been disclosed, in which instead of increasing the Si content, the Al content in the steel is positively increased to thereby prevent the surface quality deterioration peculiar to the Si-added steel and simultaneously make the retained austenite stable (JP-A-5-171344).
However, because Al and Si are readily oxidizable elements, in addition to the Si oxide film, an Al oxide film is formed during annealing, and as with the Si-added steel, it has been impossible to prevent deteriorations in the hot-dip galvanizability and the galvanized coating adhesion in the Al-added steel sheet.
It is generally well known that Al is an element to deteriorate weldability. To make an Al-added steel practically applicable, it has become an imperative requirement to improve its spot-weldability.
When a high-tensile steel sheet is used for automobiles, after chemical treatment or electrodeposition coating, a top coat is applied when necessary, and as demand is mounting for rust resistance in recent years, the improvement of corrosion resistance after an electrodeposition process is increasingly important. However, a high-tensile steel sheet, which contains a large amount of galvannealing elements with high reactivity, is poorer in corrosion resistance than mild steel. For this reason, if one tries to further improve corrosion resistance, there is a problem of difficulty in increasing high strength.
The present invention has been made to solve the above problems in the prior art, and has as its object to provide a coated steel sheet superior in coating adhesion even if the base sheet is an Al-containing steel sheet, and also provide a method for manufacturing this coated steel sheet.